Fan with adjustable inlet for secondary air



Jan. 12, 1954 E. M. POWELL 2,665,840

FAN WITH ADJUSTABLE INLET FOR SECONDARY AIR Filed Sept. 1, 1949 2 Sheets-Sheet l INVENTOR Elno M. Powell Jan. 12, 1954 E. M. POWELL 2,665,840

FAN WITH ADJUSTABLE INLET FOR SECONDARY AIR Filed Sept. 1, 1949 2 Sheets-Sheet 2 INVENTOR I Elno M. Powell ATTORNEY Patented Jan. 12,, 1954 FAN 'WITHA'DJUSTABBEINLET. FORK SECONDARY AIR Elno. M. Powell; Seven Bridges, NZ Y., assignor to;

Combustion Engineering,

Inc., New York, N. Y.,

a corporation'of Delaware Application September 1, 1949, Serial No. 113,587

1 Claim.

This invention relates to inletsfor'admitting secondary air into fans; one illustrative application being to the hot air inlet of a forced draft fan associated with an air heater utilized by a steam generating unit in such a way that the flue gases leaving the air heater are at times cooled to relatively low temperatures.

In the conventional air heater when so utilized the heating gases and cold air to be heated usually flow in countercurrent relation to each other with the result that the heat absorbing surface at one end of the heater is swept both by cold air and by cooled gases. This results in low metal temperature of this heat absorbing surface at the cold heater end where air enters and Where gas is discharged. In air heaters in which the heat ing gases are to be cooled to a low temperature to attain a high efilciency of heat recovery, the metal temperatures at the'gas outlet end may be so low as to be below the dew point of the gases and thereby cause corrosion due to condensation of moisture in the presence of gases containing sulphur oxides.

This is especially true at periods of low load operation. Then it frequently becomes necessary to recirculate hot air from the air heaters hot air end to the inlet of the forced draft fan supplying cold air to the cold airv inlet of the air heater. By recirculating hot air in thisinanner the temperature of the air entering the air'h'eater can be increased to'a point where: danger of condensation of the. moisture in the flue: gases upon the air heater heating surface. is avoided, thereby reducing or eliminating corrosion of these heating surfaces.

In delivering the hot or secondary air to the fan it has heretofore been the practice in the prior art to discharge such air freely infront of the axial bell-shaped fan inlet, said secondary airbeing drawn into the fan together with. cold room or primary air by the suction of the fan. However in cases where the fanbearing and motor are located close to the fan inletth'e'. high. temperatures of the hot secondaryair has-heen found to cause overheating and eventual failure or the bearing, as well as overheating and consequent restriction of overload margin of fan motor.

It also was found, in' operating theseunits under such conditions of freelrot air discharge, that operating personnel passing in front of the fan inlet were in danger of being, burned by coming in contact with this hot recirculated. air, the temperature of which may at times reach more than 600' F. Furthermore when operating in conjunction with a regenerative air heater a considerable dust nuisance was created by. conveyance of dust. from the gas stream to the air stream through revolving air heater surfaces and deposit of said dust by way of air recirculation in the room occupied by the fan and motor.

For these reasons a free discharge of hot air in front of'the fan inlet was found to be undesirable and a more advanced design was proposed incorporating a tight connection between the duct terminal and the fan inlet casing. Although this design afforded. full protection to the fan bearing against excessive heat emanating from the hot. or. secondary air, such a connection also considerably reduced the eificiency of the fan during operation periodswhen secondary air was not being. required.

The cause of this low fan efficiency was that at such loads, which usually coincide with high steam generating loadswhen" a high fan capacity is required,. only part of the fan inlet was available for passage of. the incoming air since that portion of theair inletwconnected to the hot air recirculating. duct could. not be utilized for admitting cold. primary air into the fan. Said portion then, in theearlier improved design, constituted: an obstruction disturbing the original flow pattern-of the. air stream entering the fan; this original pattern having been established by careful design of the. fan inlet in order to keep entrance losses low and fan operating efficiency high..

My invention overcomes the foregoing difficulty of' uneven air now in a practical and inexpensive way without sacrificing any of theadvantages (such as protecting the fan bearing from the heat of the secondary air) inherent in the aforesaid conventional design of fan air inlet.

It is accordingly an object of my invention to provide a simple and inexpensive construction enabling admittance of. hot or secondary air to a portionof' the faninlet during certain operat ing periods: and permitting this same portion or area of thefan inlet tobe utilized for cold primary air admission. during other operation periods when hot air recirculation is not required.

A further object is to provide a combined duct and. fan inlet construction permitting cold room air or cold room air. tempered. with hot recirculated air'to be drawn into. the fan inlet without impairing the: operating e'fiiciency' of the fan or overheating; the; bearing thereof, when. using either hotair'or room. air.- or a mixture of both.

These and other objects of theinvention will become apparent to those. skilled. inthe art from the following description of illustrative embodiments of the invention when taken in conjunction with the accompanying drawings wherein:

Fig. 1 represents a diagrammatic cross section of a typical hot air recirculating system showing air heater, forced draft fan and recirculating duct terminating in a fan air inlet which incorporates the improvements of the invention;

Fig. 2 is an enlarged cross sectional elevation taken on line 2-2 of Fig. 3 of the hot air duct of Fig. 1 showing further details of how same is connected to the annular fan inlet and how same utilizes vane mechanism organized in accordance with this invention; and

Fig. 3 is a side elevation of the Fig. 2 apparatus taken on line 3-3 of Fig.2.

Illustrative apparatus to be benefitted Referring first to Fig. 1, the main features of a hot air recirculating system are there shown heater through flue It. Forced draft fan B when driven by motor it draws in cold air through axial inlets l3 and delivers this air through cold air duct 28 to air heater A, here illustratively shown as having a plurality of air passes formed by baifies 22 through which passes the air moves up and to and fro around they gas tubes is and leaves the hot end of theair heater by way of duct 24.

Low steam load operation As explained above, at times, during periods of low steam loads, it becomes necessary in order to avoid corrosion of air heater heating surface to recirculate a portion of the hot air through duct C from which it enters fan inlet is where it mixes with room or primary air, drawn into the fan simultaneously; thereby elevating the metal temperature of the air heater tubes adjacent the cold air inlet end 23 of the air heater above the dew point of the gases. Recirculation is made possible by the positive static pressure existing at the inlet end 23 of duct C and the negative static pressure created by the fan at the terminal end 35 of duct C (see Figs. 1 and 2).

By opening shut-off damper 32 recirculating flow of secondary airis established, and is con trolled by manipulation of a novelly organized pivoted wall portion or vane "i i (see Fig. 2). Such manipulation may be accomplished in any suitable manner as through the medium of a hand wheel 33 geared to turn the shaft 35 by which the adjustable vane 36 is carried. Dot and dash lines indicate extreme position (a) occupied by vane t l when the full amount of hot air is recirculated and at which position said'van'e E l forms an integral part of duct wall '36 of duct 0. Hot or secondary air then flows through a segmental portion 38 (see Fig. 3) of the axial fan inlet All; the remaining portion of fan inlet it being utilized to draw in cold room'or primary air. The extreme end portion of "duct wall 35 terminating adjacent the fan inlet casing it affords to the fan bearing lt complete protection from hot air spilling overbearing as well as from heat radiation.

Operating under the above described conditions with full hot air recirculation it can be seen that substantially the total area of the axial fan inlet lib is being utilized for passage of air into the fan making it possible, due to equal air distribution over the whole inlet area, to realize minimum entrance loss and maximumian efficiency; at the same time protecting the fan bearing against excessive heat absorption;

High steam load operation Consideration will now be given to the other extreme operating conditions which usually prevail when the steam generating unit is evaporating steam at or near its maximum capacity. Then themetal temperature of the air heater tubes 52 at the cold end 26 of the air heater A is generally above the dew point of the gases due to a higher exit gas temperature, thereby greatly reducing or entirely eliminating the danger of condensation and the resulting corrosion of these metal surfaces by the presence of sulphur oxides in the flue gases. Under these conditions it becomes unnecessary to recirculate hot air through duct C in order thereby to raise said metal temperature. To accomplish cessation of secondary air flow, pivoted wall portion 3d is adjusted to occupy the other extreme position (2)) thereby blocking passage through said duct C complete- 1y. Should this condition prevail for any length of time shut-off damper 32 can also be closed.

Summing up the problem to be solve-cl With the growing economic importance of a high heat recovery in the design of steam generating units and the resulting lowering of gas exist temperatures it becomes equally important to solve the air heater maintenance problem created by heating surface corrosion at low operating loads. While prior art design successfully solved this problem by recirculating hot air, considerable difficulties with respect to efficient fan operation became apparent as soon as an attempt was made: (1) to eliminate overheating of fan bearings and motor due to exposure to hot recirculated air; (2) to eliminate the danger of burning passers-by; and (3) to prevent resultant dust nuisance from spillage of hot air when using a regenerative air heater.

It is a well known fact in the art of designing steam generating plants that the exit-gas temperature increases at high operating loads making it economically undesirable as Well as unnecessary from the corrosion problem standpoint to recirculate hot air around the air heater. Consequently the portion of the fan inlet occupied by the terminal of the hot air duct constituted an obstruction to the free and even flow of air into the forced draft fan, thereby lowering the fan efiiciency considerably and making it necessary to use more power for the delivery of a given amount of air.

With existing equipment such lowering of the fan efficiency of course causes a decrease of fan capacity at top load resulting in a curtailment of steam generating capacity of the whole unit or a narrowing of the conventional margin of safety usually provided when designing such equipment. Both limitations are of course exceedingly undesirable from the standpoint of overall plant efficiency and partly oiiset the advantages gained by recirculating hot air at lower loads.

My invention of providing a pivoted wall section 34 at the location indicated in the illustrative embodiment successfully overcomes these difliculties. This has been substantiated by operating experience in actual plant installations.

Advantages of invention By the novel arrangement and novel location of pivoted wall section 34 with respect to duct exit and fan inlet, in accordance with my invention, it is possible to open up the exit end portion of duct 0 to cold room or primary air simultaneously and automatically when hot or secondary air flow is restricted or stopped by the positioning of pivoted wall section 34. Such novel arrangement makes it possible to fully utilize the whole area of the fans axial inlet regardless of which position pivoted wall section 34 occupies, since either hot secondary air above the pivoted wall section or cold room primary air below the pivoted wall section or both will always be available to be drawn into the segmental portion of the axial fan inlet, forming at the same time the exit of circulating duct C.

While in the foregoing description I have set forth in illustrative detail one embodiment of the invention, it will be understood that design details of the structures may be varied widely by those skilled in the art without departing from the spirit of my invention.

What I claim is:

In a fan having a casing provided with an axial inlet of circular cross section through which primary air is drawn, an inlet box surrounding a selected portion of said axial inlets area, said box having a bottom wall generally parallel to a transverse axis normal to the axis of rotation of said fan, two opposing side walls, and a roof wall generally conforming to the peripheral contour of said axial inlet; a fan journal bearing positioned adjacent said axial inlet but outside of said selected portions inlet box; a duct cooperating with said inlet box walls for supplying secondary hot air into said selected inlet portion and including a duct wall forming a continuation of the bottom wall of said inlet box and interposed between the selected inlet portion and said journal bearing, whereby the bearing is shielded from and kept free of contact with said secondary air; said duct also including duct walls forming a continuation of the side walls and roof wall of said inlet box whereby hot secondary air delivered through said duct is confined to said selected portion inlet and prevented from spilling outside said fan casing and thereby constitute a hazard to operating personnel; said duct having a pivoted wall portion which is adjustable from one position in which substantially the entire area of said inlet including said selected portion is available for entrance of primary air and adjustable to another position in which said entire area of the inlet is available for entrance of primary air with the exception of said selected inlet portion through which secondary air from said duct then enters the fan, whereby substantially the total area of the fans said axial inlet remains available for admission of air into the fan regardless of the position of said pivoted wall portion and the amount of secondary air being drawn through said duct.

ELNO M. POWELL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 871,541 Wallace Nov. 19, 1907 1,563,963 Byrd Dec. 1, 1925 2,084,408 Mueller June 22, 1937 2,109,240 Scholl Feb. 22, 1938 2,198,449 Atkins Apr. 23, 1940 2,232,587 Brandt Feb. 18, 1941 2,287,595 Beckett June 23, 1942 2,363,870 Karlsson et al Nov. 28, 1944 2,429,619 Hamblin Oct. 28, 1947 2,476,543 Geissler et al July 19, 1949 2,488,267 Cassie Nov. 15, 1949 2,496,540 Holmes et al Feb. 7, 1950 2,499,358 Cooper et a1 Mar. 7, 1950 

